Android设备如何保证数据同步写入磁盘

在一些特定的工作场景中，我们把数据及时写出磁盘，而不是暂时保存在系统的文件缓存区，防止掉电导致数据丢失

/** * Force all system buffers to synchronize with the underlying * device.  This method returns after all modified data and * attributes of this FileDescriptor have been written to the * relevant device(s).  In particular, if this FileDescriptor * refers to a physical storage medium, such as a file in a file * system, sync will not return until all in-memory modified copies * of buffers associated with this FileDescriptor have been * written to the physical medium. * * sync is meant to be used by code that requires physical * storage (such as a file) to be in a known state  For * example, a class that provided a simple transaction facility * might use sync to ensure that all changes to a file caused * by a given transaction were recorded on a storage medium. * * sync only affects buffers downstream of this FileDescriptor.  If * any in-memory buffering is being done by the application (for * example, by a BufferedOutputStream object), those buffers must * be flushed into the FileDescriptor (for example, by invoking * OutputStream.flush) before that data will be affected by sync. * * @exception SyncFailedException *        Thrown when the buffers cannot be flushed, *        or because the system cannot guarantee that all the *        buffers have been synchronized with physical media. * @since     JDK1.1 */public native void sync() throws SyncFailedException;

   可能一看到这个场景，很多人会想到数据库的事务，查看Android数据库sqlite的源码可以看到，数据库事务只能保证n个操作，要么都执行，要么都不执行。数据库事务在所有操作完成后，会提醒文件系统与磁盘同步，但是不会等到所有系统缓冲区与磁盘同步完成才返回！   FileDescriptor.getFd().sync();会强制所有系统缓冲区与磁盘同步

File file = new File("/sdcard/a.txt");        try {            FileOutputStream outputStream = new FileOutputStream(file);            outputStream.write("kuangxf".getBytes());            outputStream.flush();            //强制文件系统刷新            outputStream.getFD().sync();        } catch (Exception e) {            e.printStackTrace();        }

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